About interstitial ads
Interstitial ads present a full page ad experience at natural transition points such as a page change, an app launch, or a game level load. Interstitials use a close button that removes the ad from the user's experience. Ads are displayed in an iFrame that floats over the web page or app. By default, all of the pages that make up an ad appear at the top left corner (coordinates 0,0) of the enclosing iFrame, but this can be modified by adding an offset.
If you're designing an ad that's meant to be shown on several different devices with different screen sizes, you should design your ad to fill the largest screen, but you should confine the important content to the area defined by the smallest screen the ad will be shown on. Then you can create the ad with a background image that can show more or less depending on the device size.